Quasi-epitaxial heterojunction interface reconstruction enables 11.96% certified efficiency in Cd-free Cu ₂ ZnSnS ₄ solar cells

(CZTS) is a promising photovoltaic material with considerable potential for application in single-junction, multi-junction, and tandem solar cells. However, advancements in its certified power conversion efficiency (PCE) have remained slow, primarily due to severe interface recombination, which is a major limitation constraining the overall performance of CZTS solar cells. Here, we report an effective heterojunction interface engineering approach in which the CZTS/(Zn,Sn)O heterojunction is subjected to a controlled annealing treatment under dynamically flowing air atmosphere. This treatment induces Zn and Sn cation lattice diffusion to reconstruct a quasi-epitaxial contact at the intermediate interface. This reconstruction effectively suppresses defect-assisted interfacial carrier recombination and optimizes carrier dynamics by enhancing transport and collection efficiencies. Consequently, we achieved a highest certified efficiency of 11.96% for Cd-free, pure-sulfide CZTS solar cell (bandgap > 1.5 eV) without extrinsic cation alloying. This study provides insights into heterojunction interface optimization and the performance enhancement mechanism in the development of kesterite solar cells.